1. Field of the Invention
The present invention relates to a method for controlling communication resources in a cellular communication network, wherein the cellular communication network comprises a plurality of cells under the control of at least one base station, and wherein the communication resources are frequency resources and/or time resources. The present invention further relates to a control unit implementing said method, and to a base station and a user equipment for use in said method. Particularly, but not exclusively, the present invention relates to cell resource allocation with the LTE (Long Term Evolution) and LTE-Advanced radio technology groups of standards as, for example, described in the 36-series (in particular, specification documents 36.xxx and documents related thereto), releases 9, 10 and subsequent of the 3GPP specification series. However, the present invention is also applicable to UMTS, WiMAX and other communication systems in which cell resources may be allocated.
2. Description of Related Art
A cellular communication network refers to a radio communication network comprising several geographical areas which are called “cells”. The term “cell” generally refers to a radio network object as a combination of downlink and optionally uplink resources. A cell can be uniquely identified by, for example a user equipment (UE), from a (cell) identification that is broadcasted over the geographical area from an Access Point or base station. A cell may be in FDD (Frequency Division Duplex) or TDD (Time Division Duplex) mode, thus communicating with the user equipments assigned to the cell using frequency or time as communication resources. Examples of cellular communication networks are UMTS (Universal Mobile Telecommunications System), LTE, LTE-Advanced, WiMAX, also referred as “4G”, and the like.
FIG. 1 illustrates an example of a cellular communication network. In FIG. 1, a plurality of cells A to Q are depicted. For the sake of simplicity, each cell is assumed to have the shape of a hexagon, thus resulting in a honeycomb cellular communication network, although the actual shape of a cell may differ. In this example, each cell is subdivided in a cell centre area depicted as a circle in the centre of the cell and a cell edge area surrounding the cell centre area. The cells are adjacent to each other in their respective cell edge areas, which is also referred to “inter-cell area”.
In FIG. 1, each cell is under control of one base station. However, a base station may also control a plurality of cells. A user equipment (UE) is illustrated in the inter-cell area (marked as a dotted line in FIG. 1) of cells G and J, thus being able to communicate with the base station of cells J and/or G depending on the signal quality of the communication link with the respective base station.
The cellular communication system of FIG. 1 employs Orthogonal Frequency-Division Multiple Access (OFDMA). OFDMA is a multi-user version of the Orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers (also referred as “communication resources”) to individual user equipments. By assigning distinct frequency/time communication resources to each user equipment in a cell, OFDMA can substantially avoid interference among the users served within a given cell. In many cellular communication systems, including those based on OFDMA, the same set of frequency domain resources can be allocated to every cell.
In order to reduce inter-cell interference, a fractional frequency re-use (FFR) has been proposed. FFR schemes divide the frequency spectrum into sub-sets, a cell centre set and a cell edge set of communication resources. According to FFR, the cell centre set is identical for all cells, and the cell edge set is different for adjacent cells. Thus, inter-cell interference for UEs in the edge area can be reduced, as these UEs communicate on different communication resources. However, since FFR does not use the whole available frequency bandwidth in each cell, in the absence of significant adjacent channel interference (e.g. due to low load in adjacent cells), overall cell throughput in a cell employing FFR is lower than in a cell with a re-use factor one, i.e. a cell serving the UE with the whole available frequency bandwidth.
A soft FFR scheme has been proposed to increase the overall cell throughput of cells employing the FFR scheme. Soft FFR assigns communication resources of the cell edge sets in cell centre sets, but with low transmit power. Thus, more communication resources may be employed in soft FFR while maintaining a re-use distance for the same communication resources by appropriately selecting the cell centre sets and the transmit powers.
FIG. 1 illustrates how a soft FFR scheme may be applied to a cellular communication network using six frequency resources designated as f1 to f6. A frequency re-use 3 pattern is applied at the cell edges, with two frequency bands employed for each cell edge. The cell centre areas occupy the other 4/6 of the frequency bands, (frequency resources). Assuming uniform user distribution in a cell, the cell centre areas span 2/3 of the whole cell area or 82% of the cell radius. The power allocation for the cell centre areas may be approximately 3 dB down from the cell edge power allocation, assuming a path loss exponent of 3.5. A vertical antenna pattern with 6° half power beamwidth may be employed.
FIG. 2 illustrates the signal to interference ratio (SIR) as seen by the user equipment at the inter-cell area of the cellular communication network of FIG. 1. The SIR is calculated for random user equipment locations in the inter-cell area and the shadow fading component is varied, according to a log-Normal distribution with 8 dB standard deviation. The user equipment uses the communication link as per, for example, the best measured RSRQ (Reference Signal Received Quality) value as defined in LTE standard document TS 36.331, V10.0.0, section 5.5, which is hereby incorporated in its entirety by reference. The number of occupied sub-carriers of the frequency resources is varied according to the load of each cell and the number of collisions with the cell edge user equipment is calculated. Each iteration sees a random allocation of sub-carrier indices for the cell edge userequipment and neighbour cells, depending on their load. The simulated SIR for the 10,000 iterations is plotted in FIG. 2.